The present invention relates to a process for separating alkali metal ions from alkoxylates containing alkali metal ions, alkali metal-free alkoxylates and a process for the preparation of alkali metal-free alkoxylates.
Alkoxylates, in particular polyalkylene oxides and adducts of alkylene oxides with alcohols and/or alkylphenols, are usually prepared under alkali metal hydroxide catalysis. Depending on the intended use, it is frequently necessary to remove the catalyst as completely as possible from the adduct. This is the case, for example, with alkoxylates which are used as fuel additives or carrier oils in fuel additive packets or formulations. Such alkoxylates for carrier oils are in general adducts of propylene oxide and/or butylene oxide with alcohols and/or alkylphenols of more than 6 carbon atoms, which are prepared by catalysis using potassium hydroxide. In order to ensure substantially residue-free combustibility of the carrier oils, the catalyst must be separated off. In the conventional processes, this is done by neutralization and precipitation as acidic potassium phosphate and subsequent filtration. After the synthesis of the alkoxylates, also referred to here generally as polyethers, it is therefore necessary to neutralize the potassium alcoholate contained in the product with dilute phosphoric acid (stoichiometric amounts of phosphoric acid dissolved in about 10%, based on the reactor content, of water) and to distill off the water for crystallization of the acidic potassium phosphate. The reactor content must then be filtered, for example through a batchwise sheet filter, which is manually loaded and scraped off. Further required steps are the separation and separate packing of product-moist salt and impregnated filter sheets, their transport and incineration; the cleaning of the reactors before the subsequent batch, also in the case of a batch procedure, in order to remove remaining phosphate residues which neutralize marked amounts of catalyst and can thus delay or even suppress initiation of the oxyalkylation reaction; the drying of the reactors for the subsequent batch. It is clear that the removal of the catalyst is expensive. Moreover, the carrier oils thus obtained still contain small amounts of potassium and also phosphorus, so that residue-free combustion of the carrier oils is not possible.
It is an object of the present invention to provide alkoxylates which are substantially free of catalyst impurities from the preparation.
It is a further object of the present invention to provide a process for the preparation of alkoxylates which are substantially free of catalyst impurities from the preparation.
It is a further object of the present invention to provide a process for separating the catalyst from alkoxylates, which permits substantial removal of the catalyst from the product and preferably avoids contamination of the product with phosphate.
We have found that this object is achieved by the novel process for the separation of alkali metal ions from alkoxylates containing alkali metal ions (in particular potassium and sodium ions), which comprises the following steps:
a) dilution of the alkali metal-containing alkoxylate with an inert solvent,
b) treatment of the alkali metal-containing solution of the alkoxylate with a cation exchanger for exchanging alkali metal ions for hydrogen ions in order to obtain a substantially alkali metal-free solution of the alkoxylate, and
c) removal of the solvent from the substantially alkali metal-free solution of the alkoxylate in order to obtain a substantially alkali metal-free and substantially solvent-free alkoxylate.
In the novel process for the preparation of alkoxylates, the alkoxylates are initially prepared in a conventional manner, and the catalyst is then removed by the novel process for separating off alkali metal.
The term alkali metal-free or substantially alkali metal-free means that less than 5, preferably less than 1, ppm of alkali metal ions are present. The alkali metal-containing alkoxylate to be purified generally contains from 5 000 to 100, in particular from 2 000 to 1 000, ppm of alkali metal ions.
The term substantially solvent-free means that the alkoxylate contains  less than 1 000, preferably  less than 500, ppm of solvent.
The term alkoxylate includes pure substances as well as mixtures which are obtained using different alkylene oxides and/or different alcohols.
The term alkoxylate includes polyalkylene oxides (polyethers) and alcohol- and/or alkylphenol-initiated polyethers. The polyether or the polyether moiety of the alcohol- and/or alkylphenol-initiated polyethers is generally composed of at least one C2-C6-alkylene oxide, in particular ethylene oxide, propylene oxide, n-butylene oxide, 2,3-butylene oxide and/or isobutylene oxide. In general, at least one C1-C50-alkanol, preferably C2-C20-alkanol, particularly preferably C6-C14-alkanol, in particular 2-ethylhexanol, nonanol, isononanol, tridecanol, isotridecanol, etc., is used as the alcohol. The alkylphenol used is in general a C1-C50-alkylphenol, particularly preferably a C6-C14-alkylphenol, preferably a C6-C14-alkylphenol, in particular nonylphenol, octylphenol or dodecylphenol, or a di-C1-C50-alkylphenol.
Alkanol-initiated polyethers having from about 10 to 35, preferably from about 15 to 30, alkylene oxide units are preferred.
The preparation of alkoxylates is known per se. Polyether syntheses are described, for example, in Ullmann""s Encyclopedia of Industrial Chemistry, 5th Edition, Vol. 21, 1992, 579-589, and the publications stated therein. The preparation of alcohol- or alkylphenol-initiated polyethers is described, for example, in Ullmann""s Enzyklopxc3xa4die der technischen Chemie, 4th Edition, Volume 22, 491-492 and Volume 19, 31-33.
The catalyst-containing crude alkoxylate product is initially diluted with an inert solvent for removal of the catalyst. Solvents used are in general an aliphatic or cyclic ether, such as tert-butyl methyl ether, tetrahydrofuran or dioxane, a hydrocarbon, such as pentane, hexane, toluene or xylene, a ketone, such as acetone or methyl ethyl ketone, and preferably an alcohol, in particular a C1-C4-alkanol, such as ethanol, isopropanol, n-butanol, isobutanol and preferably methanol. For removal of the alkali metal catalyst, the dilute solution is treated with a cation exchanger, for example is passed through an exchanger bed, in particular in the form of a column, or is stirred with the cation exchanger. Particularly suitable cation exchangers are strongly acidic, macroporous resins, for example those based on crosslinked polystyrenes having sulfonic esters as functional groups.
The amount of cation exchanger required for removal of the catalyst is dependent on the catalyst content of the product to be treated and on the capacity of the ion exchanger used.
The solvent is then removed again, for example by distilling off. The removal is preferably effected in two steps. In a first step, the main amount of the solvent is removed, preferably by distilling off, an alkoxylate solution depleted of solvent and the solvent being obtained. In the first step, preferably at least 80% and up to 95% of the solvent are removed. In a second step, the remaining amount of the solvent is removed, preferably by stripping the depleted solution of the alkoxylate with inert gas in a column, in order to obtain a substantially alkali metal-free and substantially solvent-free alkoxylate and alcohol.
After a specific operating time, the cation exchanger needs to be regenerated. The regeneration is preferably integrated in the overall process, i.e. alkoxylate solution still contained in the cation exchanger is recovered before the regeneration and is recycled to stage c) or a) of the catalyst separation process. Any residues of the alkoxylate solution which still adhere to the cation exchanger are removed by washing with the inert solvent. The wash solvent is likewise recycled to stage a) of the catalyst separation process.
The regeneration of the cation exchanger preferably comprises the following steps:
d1) removal of the alkoxylate solution from the cation exchanger and, if required, washing of the cation exchanger with the inert solvent; this can be effected in such a way that the alkoxylate solution is removed, for example by discharging, and the cation exchanger is then washed with the solvent; alternatively the solvent can be fed in without prior removal of the alkoxylate solution, until the alkoxylate has been washed out,
d2) if required, washing of the cation exchanger with demineralized water,
d3) regeneration of the cation exchange resin with an acid, preferably sulfuric acid,
d4) washing the cation exchange resin neutral with demineralized water,
d5) washing out the water present in the ion exchanger resin with an inert solvent, preferably a water-miscible inert solvent, and
d6) if required, loading of the cation exchanger with the inert solvent desired for the treatment with the cation exchanger.
The inert solvent used in the regeneration of the cation exchanger is preferably the same as that also used in the catalyst separation process. Step d6) is then omitted.
Preferably, the alkoxylate adducts are stripped with steam or an inert gas, such as nitrogen, after the synthesis, i.e. before step a) of the novel process.
The present invention can be particularly advantageously used for separating potassium ions from adducts of ethylene oxide and/or propylene oxide and/or butylene oxide, in particular propylene oxide and/or butylene oxide, with C6-C14-alcohols and/or C6-C14-alkylphenols, the inert solvent used preferably being a C1-C4-alkanol, in particular methanol.
A preferred embodiment of the present invention is described below. The stated amounts of solvent/diluent and temperature ranges are preferred values for separating potassium from propylene oxide/butylene oxide adducts using methanol as a diluent. They may assume different values in the case of other adducts, catalysts and solvents, but the optimum values can be readily determined by a person skilled in the art using routine methods.
For dilution of the potassium-containing adduct (step a), preferably from 5 to 25, particularly about 15, % (m/m) of methanol are added to the adduct. The methanolic solution is then treated with a cation exchanger, for example is passed over an ion exchanger bed which contains a cation exchanger. It is possible to use a commercial ion exchange resin, preferably in granular form. For example, the abovementioned ion exchangers, for example Lewatit SP 120 (Bayer) and Amberlite 252 C (Rohm and Haas), are suitable.
The service life of the cation exchanger is in general 1 year or longer. On passing through the ion exchanger, the methanolic solution is preferably at from about 20 to 60xc2x0 C., particularly preferably about 50xc2x0 C. The cation exchanger is present in the acidic form. During the passage of the potassium-containing methanolic solution of the adduct it binds potassium ions and releases protons according to the following equation:
Rxe2x80x94Oxe2x80x94(CHRxe2x80x2xe2x80x94CHRxe2x80x3xe2x80x94O)xxe2x80x94K++IEXH+Rxe2x80x94Oxe2x80x94(CHRxe2x80x2xe2x80x94CHRxe2x80x3xe2x80x94O)xxe2x80x94H++IEXK+
IEX=ion exchanger
R=alkyl or alkylaryl
Rxe2x80x2xe2x95x90Rxe2x80x3=H, CH3, C2H5 
After the treatment with the cation exchanger, the methanolic adduct solution is greatly depleted of potassium, in particular substantially potassium-free, and particularly preferably the concentration of the potassium ions is not more than 1 ppm.
For separating off possible fine fractions of the ion exchange resin, the solution is then filtered in a conventional manner. Before the further treatment, it may be temporarily stored in a container.
It is advisable for the potassium content of the adduct solution leaving the ion exchanger to be monitored continuously or at least at regular intervals by means of analytical measurement. In the event of a breakthrough of potassium ions the exchanger must be regenerated. This is done by means of an acid, preferably sulfuric acid, particularly preferably about 5% strength sulfuric acid, by the following procedure:
First, the feed stream of the potassium-containing methanolic solution of the adduct to the ion exchanger is stopped. Then, methanol can be fed in in order to wash the ion exchanger product-free. Adduct-containing methanol, which in turn is used for diluting the crude product, i.e. the catalyst-containing alkoxylate, is obtained. However, before the washing with methanol, the potassium-free adduct solution still present in the ion exchanger is preferably forced out of the ion exchanger with nitrogen or another inert gas and is combined with the other potassium-free adduct solution for the further treatment. The ion exchanger is then washed product-free with methanol, and the wash methanol leaving the ion exchanger and containing a small amount of adduct is used for diluting the potassium-containing crude product. Preferably, the wash or rinse methanol is also preferably filtered to separate off possible fine fractions of the ion exchange resin before being used further.
The potassium-laden ion exchanger is now full of methanol, which has to be removed before the regeneration. One possible method is to wash the ion exchanger with demineralized water by the countercurrent or cocurrent method. The methanol-containing wastewater obtained here is not used further but is disposed of via a wastewater treatment plant. In order to avoid relatively large losses of methanol, it is, however, preferable to remove the methanol, for example to force it out of the ion exchanger by means of an inert gas, such as nitrogen, before washing the ion exchanger with demineralized water. This methanol is preferably combined with the other, adduct-containing rinse methanol and is used again for diluting subsequent batches of potassium-containing adducts.
The ion exchanger is then converted back into the acidic form by passing through dilute sulfuric acid (1 to 20% by weight), preferably about 5% strength sulfuric acid, by the countercurrent method. This is followed by washing neutral with demineralized water, and the water is finally washed out with methanol, preferably by the trickle-bed procedure. The resulting aqueous methanol phase can be used instead of fresh methanol for preliminary cleaning of reactors on product change.
After complete replacement of the water by methanol, the ion exchanger is ready for operation again and can be loaded again.
The substantially potassium-free adduct solution leaving the ion exchanger is further treated as follows:
For reasons relating to application technology, the solution must be freed from the solvent as completely as possible. This is preferably carried out in an evaporator unit, in particular a single-stage one, with connected stripping column. The preferred temperature range is from about 150 to 170xc2x0 C., particularly preferably 160xc2x0 C. First, the main amount of the methanol, preferably at least 80%, is separated off by distillation. The recovered methanol can be collected and reused.
In particular an inert gas stripping column in which the concentration is reduced to methanol contents of less than 1 000 ppm, preferably less than 500 ppm, is used for removing the remaining methanol from the adduct solution. The inert gas is preferably nitrogen.
The methanol stripped off in the stripping column can also be reused. The substantially potassium-free desired products having a low methanol content and leaving the stripping column are preferably passed through a heat exchanger, where they preheat the adduct solution having a low potassium content, before entry into the evaporator apparatus and are themselves cooled to about 50 to 60xc2x0 C.
The novel process comprises a plurality of process steps in which pure methanol is required, i.e. in washing the ion exchanger free of product, in washing the water out of the ion exchanger and in filling the ion exchanger with methanol after the regeneration and, if required, for diluting the crude product prior to separating off the potassium in the ion exchanger. The methanol distilled off and stripped off is preferably reused for this purpose. For diluting the potassium-containing crude product, adduct-containing methanol from the washing out of the ion exchanger is preferably additionally or exclusively reused.